As methods for manufacturing a metal pipe, there is a method of manufacturing a seamless pipe by directly making hole into a metal billet, or a method of manufacturing a pipe by extruding a metal, in addition to a method of manufacturing an electric-resistance-welded pipe, a spiral pipe, or the like, in which a metal strip is bent and welded into a pipe shape.
Because of the particularly high productivity and the low cost, electric-resistance-welded pipes have been produced in large quantities. Such electric-resistance-welded pipes are molded into a cylindrical shape while the metal strip is travelling, and finally, high frequency current flow is applied to both ends of the metal plate to be joined, so that the temperature of both ends are increased to the melting temperature, and the both end surfaces are press-welded by rolls, resulting in a pipe shape. At this time, as methods for supplying current to both ends of the metal strip, there is a method in which an induction coil is wound so as to surround the outside of the metal strip and the induction current is directly generated in the metal strip by making a primary current flow through the induction coil. As another method, there is a method in which metallic electrodes referred to as a “contact tip” are pushed against the ends of the metal strip and the current flows directly from a power source to the metal strip. At this time, in many cases, as the current which flows through the induction coil or the electrodes, high frequency current having a frequency of about 100 to 400 kHz is generally used and a ferromagnetic body referred to as an “impeder” is disposed on or above the inner surface of the pipe.